1. Field of the Invention
The present invention relates to communication system employing MIMO technique with two or more antennas at the transmitting and the receiving ends.
2. Description of the Related Arts
Digital communication using multiple-input-multiple-output (MIMO), sometimes called a “volume-to-volume”, wireless link has recently emerged as one of the most significant breakthroughs in modern communications. A key feature of MIMO system is the ability to turn multipath propagation, traditionally a pitfall of wireless transmission, into a benefit for user. MIMO technology effectively takes advantage of random fading and, when available, multipath delay which ceases effectiveness of multiplying data transfer rates.
It is a common practice for MIMO-based communication system that for the data stream of interest, the contributions from other data streams are removed by signal processing by subtracting signal replica of the other data streams from the data stream of interest, i.e. compensation technique is employed (V-BLAST, MSSE-VBLAST, Zero Forced V_BLAST).
MIMO system can be defined simply as below. Given an arbitrary wireless communication system, we consider a link for which the transmitting end as well as the receiving end is equipped with multiple antenna elements. Such set up for the case when only two antennas are employed at the transmitting and the receiving sides is illustrated in FIG. 1. According to FIG. 1, there are two transmitters each having one antenna. From each transmitter, a different data stream is transmitted. The transmitter TRX0 receives 1st data stream having data D0 and transmits it. The transmitter TRX1 receives the 2nd data stream having data D1 and transmits it. Each data D0, D1 propagates in the air and is received by two receiver REC0, REC1. Each receiver REC0, REC1 has one antenna respectively. The received signal y0 of the receiver REC0 includes the transmitted signals of signal D0, D1. Also, the received signal y1 of the receiver REC1 includes the transmitted signals of signal D0, D1.
At the receiving array, the spatial signature of each of the received signals is estimated. Based on this information, a signal processing technique is then applied to separate the signals. Linear or non-linear receivers can be used in consideration of a range of performance/complexity trade-off.
Signal at the receivers can be expressed asy0=D0·h00+D1·h01  (1)y1=D0·h10+D1·h11  (2)or in the form of the vector productY=H·D, where
      H    =          [                                                  h              00                                                          h              01                                                                          h              10                                                          h              00                                          ]        ,      D    =          [                                                  D              0                                                                          D              1                                          ]        ,      Y    =          [                                                  y              0                                                                          y              1                                          ]      hij is transfer coefficient from j-th transmitter to the i-th receiver. D0 and D1 are symbols from the first and the second data streams respectively. Each transmitted symbol D0 and D1 is carrying several bits of information. (BPSK 1 bit; QPSK 2 bits; QAM-16 4 bits).
There are several conventional decoding methods as below.
The Zero Forcing Algorithm [See Non-Patent Document 1]
The first conventional decoding technique is so-called Zero-Forcing (ZF) algorithm. With ZF, the estimates of the transmitted vector are obtained at the receiver using the following equation:D{circumflex over (0)}=(H*·H)−1H*·Y=H+·Y  (3)where “+” represents the pseudo-inverse. In order for the pseudo-inverse to exist, the number of transmitting antennas must to be less than or equal to the number of receiving antennas.MMSE Algorithm [See Non-Patent Document 1]
Another approach in linear estimation theory to the problem of estimating a random vector D on the basis of observation Y is to choose a matrix G that minimizes the Mean Square Error (MMSE) given below.ε2=E└(D−D^)*(D−D^)┘=[(D−G·Y)*(D−G·Y)]  (4)
The solution can be obtained asD{circumflex over (0)}=(α·I+H*·H)−1H*·Y  (5)where α is equal to the noise dispersion. From (5) it becomes clear that the ZF approach corresponds to an MSSE solution with α equals to zero.Decision Feedback Decoding [See Non-Patent Document 1, and 2]
The Zero Forcing approach described before is viable, but superior performance is obtained if non-linear techniques are used. If the most reliable element of the transmitted vector D could be decoded and used to improve decoding of the other elements of D, the performance may increase. This is called symbol cancellation. Furthermore, ZF, or MMSE is used to perform detection process in Decision Feedback Decoding process. In other words, symbol cancellation is based on the subtraction of interference of already detected components of D from received signal vector Y.
Maximum Likelihood Decoding [See Non-Patent Document 1]
The only decoding method that is not based on calculation of matrix inverses is the Maximum Likelihood Decoding (MLD) algorithm. MLD is a method that compares the received signal with all possible transmitted vectors (modified by H) and estimates D according to the Maximum Likelihood principle. This principle can be formalized by the following equation.
In the non-patent document 3, the similar device as the present invention is disclosed by the inventor of the present invention. In this document, the technology which can be applied only to BPSK and QPSK cases is disclosed.D{circumflex over (0)}=arg min||Y−H·D||  (6)    [non-patent document 1]. A. van Zelst, “Space Division Multiplexed Algorithms”    [non-patent document 2] R. van Nee at al. “Maximum Likelihood Decoding in a Space Division Multiplexing System”    [non-patent document 3] Alexander N. Lozhkin “Novell Interactive MAP Detector For MIMO Communication”, Proc. of WPMC '04
For the above described receiving techniques simulations are performed to compare the BER performances. The result of the simulation is shown in FIG. 2. In FIG. 2 the BERs for different approaches are depicted against Eb/No per receiving antenna for an antenna configuration 2 by 2 as shown in FIG. 1. Furthermore, a QAM-16 modulation scheme is used and the data is transmitted without coding.
From FIG. 2 it can be seen that MLD has the best performance. However implementation of MLD requires an exhaustive search through all possible transmitted vectors. Therefore the load of calculation is large. Besides, the performance of the ZF is fairly bad compared to the MLD. The performance of the present invention is also shown in FIG. 2 as “Turbo 3TI” and “Turbo 4TI”. The superiority of the present invention is described in the chapter of the embodiment.